Photopolymerizable compositions and films or elements containing binder, monomer, initiator and chain transfer agent are available commercially. One important application of such photopolymerizable elements is in the graphic arts field. Elements containing such photopolymerizable layers are currently being used as electrostatic masters for analog color proofing and are considered as promising future materials to be developed for digital color proofing applications. For the analog color proofing application, a photopolymerizable layer is coated on an electrically conductive substrate and contact exposed with an ultraviolet (UV) source through a halftone color separation negative. The photopolymerizable composition hardens in the areas exposed with an ultraviolet source due to polymerization and remains in an unexposed liquid-like state elsewhere. The differences in viscosity between the exposed and unexposed areas are apparent in the transport properties, i.e., the unexposed photopolymerizable areas conduct electrostatic charge while the exposed areas are nonconductive. By subjecting the imagewise exposed photopolymerized element to a corona discharge, a latent electrostatic image is obtained consisting of electrostatic charge remaining only in the nonconducting or exposed areas of the element. This latent image can then be developed by application of a electrostatic toner to the surface. When the toner has the opposite charge as the corona charge, the toner selectively adheres to the exposed or polymerized areas of the photopolymerized element.
Photohardenable electrostatic masters are needed that duplicate the imaging characteristics of a printing press. Such electrostatic masters are known wherein the conductivity of both the exposed and unexposed areas can be controlled by introducing into a photopolymerizable composition an electron donor or an electron acceptor molecule that modify the electrical properties of the composition and provides a dot gain similar to that achieved by a printing press.
Although the use of photopolymerizable compositions in electrophotography has been demonstrated and many formulations can be imaged, it did not appear possible, to produce a photopolymerizable electrostatic master that was capable of producing both positive and negative images. Such results have been achieved with photohardenable elements which have a conductive support bearing a photohardenable layer comprising a polymeric binder, a compound having at least one ethylenically unsaturated group, an initiator, a photoinhibitor and at least one sensitizing compound. Positive and negative images are achieved depending on the exposure sequence and exposure wavelength. Such elements are extremely useful because a single element will satisfy the proofing needs of all printers regardless of whether they work with negative or positive color separations. A disadvantage of these elements is that they require two exposures to provide a positive-working electrostatic master.
In electrophotography, a photoconductive layer is charged and upon imagewise exposure to light a localized decay of the charge results so that applied toners or developers are attracted to the unexposed areas. Electrophotographic apparatus are generally designed so that by subsequent exposure to light the conductive layer is locally discharged and toner is then applied. After transfer of the toner, the photoconductive layer is cleaned. For multiple copies the steps of charging, exposing to light, toning and transferring are repeated. Substantially every print produced varies at least slightly from another since the image quality is affected by variations in charging and light levels that occur.
It has been found that the above disadvantage can be overcome by means of a photosensitive electrostatic master having as a photosensitive layer a photosensitive composition of this invention wherein a single exposure will provide a positive-working electrostatic master. Furthermore, this element provides a print-out image which provides a visual determination regarding exposure of the element and avoids double exposure of the element which could otherwise occur. The photosensitive electrostatic master after a single exposure can be charged, toned, charged, toned, etc. many times resulting in duplicate permanent images.